Biodiesel is fatty acid alkyl esters (for example, methyl or ethyl esters) derived from vegetable oil and animal fat through esterification of fatty acids or transesterification of triglycerides. In such reactions, a low-molecular weight alcohol (in most applications, methanol or ethanol because of the low steric hindrance) is added to a biolipid and the mixture is processed in the presence of a basic or acidic catalyst or lipase. Biolipid transesterification is a three-step process reacting a biolipid with a low-molecular weight alcohol in the presence of a catalyst to convert molecules of said oil or fat to fatty acid alkyl esters and glycerol. Possible low-molecular weight alcohols include methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, etc. Possible catalysts include a hydroxide or alkoxide of sodium, potassium, calcium, barium, an acid or lipase.
A base-catalyzed transesterification involves the nucleophilic attack of alkoxide at the carbonyl group of the triglyceride that generates a tetrahedral intermediate, from which the new ester and the corresponding anion of the diglyceride are evolved, followed by formation of another new ester molecule and monoglyceride anion, etc. The acid-catalyzed transesterification involves the protonation of triglyceride carbonyl group resulting in the formation of carbocation which, after a nucleophilic attack of the alcohol, produces the tetrahedral intermediate, which eliminates glycerol to form the new ester and to regenerate the catalyst H+ (Schuchardta et al., 1998, J. Braz. Chem. Soc. 9: 199-210).
Systems are known in the field, i.e., batch systems, whereby a catalyst (for example, alkaline metal methoxide) is mixed with methanol. The solution is then added to warm oil and the mixture is heated, typically to about 50-60° C. for about 2-12 hours, to allow the transesterification to proceed. Then the mixture is left to stand to allow for separation of biodiesel and glycerol, which may take up to 10 hours. In a continuous biodiesel production process, the reaction mixture is kept at high pressures (over 1,000 psi) within a pressure vessel. In such high-pressure systems, the temperature of oils and fats exceeds 50° C. Such a process requires significant energy input, heavy equipment and a lot of foundation space. In conventional methods, temperature for transesterification conducted at atmospheric pressure is limited by the methanol boiling point (64.7° C.).
In the processes mentioned above, the best approach to making biodiesel is using feedstock with low free fatty acids (FFA) content. High FFA content, in combination with the alkaline metal-catalyzed transesterification, lowers the yield producing soap stock. There are two approaches to FFA removal at different stages of biodiesel production, including: i) caustic stripping (alkali refining) of fat; and ii) a two-step acid-catalyzed esterification/base-catalyzed transesterification treatment. Caustic stripping yields feedstock that can be transesterified to fatty acid alkyl esters but results in saponification and lower yield. The two-step treatment comprises an acid-catalyzed esterification followed by a base-catalyzed transesterification, which produces a higher yield. (A. Demirba, 2008, Biodiesel: A Realistic Fuel Alternative For Diesel Engines. Springlert-Verlag London Limited, 205 pp.)
Existing technologies that find application in the processing industries are similar in concept in that they all require an input of energy to produce a final product. For example, some technologies include a pressurized homogenizer, which uses a sequential valve assembly to increase fluid pressure in the material being processed. Such a device requires a large energy input, producing a high outlet pressure, usually in excess of 5,000 psi. Since fatty acid triglycerides and lower alcohols cannot be mixed because of their poor solubility, many patents disclose stirring apparatuses. For example, U.S. Pat. No. 5,514,820 Assmann et al. teaches applying a Reynolds Number exceeding 2,300, and US Patent Application No 2003/10630097 by Hooker teaches ultrasonic cavitation in the reaction section to perform transesterification at or near atmospheric pressure. US Patent Application No 2008/12167516 by Kozyuk discloses a method comprising applying a controlled flow cavitation apparatus to a biodiesel production process in order to increase fatty acid alkyl ester yield.
Cavitation is defined as the generation, subsequent growth and ultimate collapse of vapor- or gas-filled cavities in liquids resulting in significant energy release. As understood in this broad sense, cavitation includes the familiar phenomenon of bubble formation when water is brought to a boil under constant pressure. In engineering and science, the term cavitation is used to describe the formation of vapor-filled cavities in the interior or on the solid boundaries created by a localized pressure reduction produced by the dynamic action of a liquid system.
Cavitation can occur at numerous locations in a fluid body simultaneously and can generate very high localized pressure and temperature (a few thousand atmospheres and a few thousand Kelvin). Cavitation also results in the generation of localized turbulence and liquid micro-circulation, enhancing mass transfer. Thus, mass transfer-limited reactions, endothermic reactions and reactions requiring extreme conditions can be effectively carried out using cavitation. Moreover, radicals generated during cavitation due to the homolytic dissociation of the bonds of molecules trapped in the cavitating bubbles or in the affected surrounding liquid, result in the occurrence of certain reactions.
In homogenous reactions, both the reagents and products remain in the same phase. The mechanical effects of cavitation play a smaller part in such reactions in comparison with the creation of high-energy intermediates. In heterogeneous reactions, cavitation bubbles collapsing at or near the phasic interface causes vigorous mixing. The surface area available for the reaction between the phases is significantly increased, thus improving the rate of reaction.
TABLE 1Comparison of energy efficiency for different methods.Time,Yield,/energy,MethodminYield, %kJ−1Acoustic10998.6 × 10−5Conventional with stirring180982.7 × 10−5Presented flow-through899.92.6 × 10−3
It can be seen from Table 1 that reactions that take place in a flow-through cavitation generator are about 30 times and 100 times more efficient compared to acoustic cavitation the agitation/heating/refluxing method, correspondingly.
Accordingly, there is a need for a method to carry out heterogeneous reactions that does not require a large amount of energy input. Further, there is a need for such a method that avoids potentially dangerous, high-pressure operation. The present invention fulfills these needs and provides further related advantages through the utilization of hydrodynamic flow-through cavitation and the chemical and physical reactions and process involved.